1. Field of the Invention
The present invention relates to an apparatus for optically testing objects such as security documents or labels with an optical system that detects the radiation from the test object and feeds it to a processing unit.
2. Description of Related Art
To detect diffusely scattering objects of low light intensity, for example fluorescent layers or bank notes with luminescent features, one uses optical lens systems that detect the light emitted by the object at a solid angle as great as possible.
In the special case of detecting luminescence, which is known to occur in a narrow defined spectral range, one uses special aspherical optical systems that permit the use of narrow-band interference filters and focus the filtered luminescent radiation on a succeeding detector.
These optical systems ideally have an aperture ratio, i.e. a lens diameter/focal length ratio D:f, of 1:1 to 2:1, whereby an aperture ratio of 1:1 or 2:1 means that a solid angle .THETA.=53.degree. or .THETA.=90.degree. is detected. This is because one half the solid angle .THETA./2 is connected with the aperture ratio, which is also frequently called the f-number, by the trigonometric relation (D/f)=2 tan (.THETA./2).
However the ideal aperture angle .THETA./2 of 45.degree. is almost impossible to realize in practice. The actually used aspherical lenses have a maximum aperture angle of 30.degree. to 40.degree. and an aperture ratio of about 1.7:1, which corresponds for example to a lens with a diameter of 22 mm and a focal length of 13 mm.
In the case of confined space relations, as are fundamentally found for example in the vicinity of the test object in bank note testers, such lens assemblies can therefore not be used without a compromise between aperture ratio and lens size. This is because a simultaneous reduction of focal length and lens diameter at a constant aperture ratio is usually subject to apparatus-related limits, in particular when thick lenses of short focal length are used to obtain a compact structure.
In the special case of bank note testers the lenses can often not be brought closer than about 10 mm to the test object so that the focal length of the entrance lens is defined. Also, the space relations are so confined particularly in bank note testers that even a reduction of the focal length to 8 mm and the resulting reduction of the lens diameter to about 13.5 mm (at a maximum aperture angle of 40.degree.) would not suffice. Therefore either smaller lenses are used, which reduces the f-number of the system, or, if a plurality of identical test systems are disposed side by side, the number of measuring systems is reduced to create room for larger single systems.
The f-number problem, i.e. the problem of detecting a maximum solid angle in the area of the test object with the optical system requiring little space, can be solved by using optical fibers. Optical fibers are known to have a large numerical aperture so that they detect a large part of the radiated intensity even at a small diameter.
A number of different fiber assemblies have already become known that are used as radiation receivers. Thus DE-A 25 59 430 describes a sensor apparatus consisting of two fiber bundles, with infrared radiation being guided via one fiber bundle onto the document to be tested which contains luminescent markings based on rare earths. Via the second fiber bundle the visible light emitted by the feature substance is fed to a photodiode.
Another embodiment is disclosed in EP-A 0 240 277. Here a fiber bundle consisting of a multiplicity of single fibers is fanned on the side facing the test object in accordance with the area to be scanned, while at the opposite end it is bunched into a small light-emitting surface and passes the transmitted radiation on to a detector.
EP-C 0 051 460 discloses a further variant that likewise has an end flared in accordance with the test object and a bunched end. However in this case it is a light-guiding sheet having a strip-like light entrance surface and a small circular exit surface followed directly by a photo-multiplier.
All these fiber assemblies have the disadvantage that the gain in f-number is lost again when an optical system is disposed therebehind. This is because with a straight optical fiber the angle of incidence corresponds to the angle of reflection. That is, if the fiber has an aperture angle of 70.degree. and the following optical system, as already stated, has a maximum aperture angle of 30.degree. to 40.degree. the radiation that emerges from the fiber at a greater angle than 40.degree. is lost for the signal evaluation.
This problem is even more serious when interference filters are used. The transmission curve of the interference filter shifts at greater angles of incidence due to changed path differences to shorter wavelengths. The maximum tolerable angle of incidence is about 20.degree. for a conventional interference filter. Therefore a lens system must fundamentally be disposed between the light guide and the interference filter to ensure a largely parallel beam path. As already stated, however, the adaptation between the lens system and the light guide is not optimal.